Polyurethane elastomers, which are formed from polyisocyanate prepolymers and polyether or polyester polyols, are polymers of moderate molecular weight having, according to present theory, flexible "soft segments" and polar, rigid "hard segments." The thermal and hydrolytic stability of the elastomers is improved if the hard segments comprise urea type linkages rather than urethane linkages. However, simple diamines cannot be used as "chain extenders" in place of diols to form urea linkages because the high reactivity of the diamines toward the isocyanates results in insufficient "pot life" for fabrication in available equipment.
Three modes of reducing the reactivity of the diamines toward the isocyanates in order to obtain useable pot lives have been developed. These modes are steric hindrance, electronic deactivation and blocking groups. An example of steric hindrance is the placement of alkyl groups in the positions ortho to the amine functions in aromatic diamines. The diethyl derivative of toluenediamine (TDA), which is made from ethylene and TDA, is a specific example. Less desirable properties in the elastomer and a relatively limited degree of deactivation are the disadvantages of this approach.
An example of electronic deactivation is the use of a chlorine substituted aromatic diamine such as methylenedi-o-chloroaniline (MOCA). While it provides long pot life and good elastomeric properties, MOCA has undesirable toxological properties and its use is no longer allowed in the United States.
Blocking groups are chemicals which react with reactive groups, such as isocyanate, and are later removed under the influence of processing conditions to regenerate the original reactive group. To be acceptable in an elastomer, a blocking group upon liberation must be so compatible with the polymeric structure formed that it never migrates to the surface or must be incorporated into the polymeric structure.
U.S. Pat. Nos. 3,098,830; 3,159,668 and 3,354,098 teach the reaction of a diamine with a diorganodihalogenosilane to yield diorganosilylamine polymers.
U.S. Pat. No. 3,172,874 discloses the reaction of a diisocyanate and a triorganosilylamine to yield polysilylureas which are readily hydrolyzed to polyureas.
U.S. Pat. No. 3,239,489 discloses poly-urea-silazanes prepared from the reaction of polyisocyanates and polyfunctional silazanes.
U.S. Pat. No. 3,243,475 discloses reacting a polyester, a polyisocyanate and certain reactive organosilicon compounds to yield elastomeric polyurethanes having improved hydrophilic characteristics and useful in making automobile tires.
U.S. Pat. No. 3,337,510 teaches the reaction of diisocyanates with bis(organosilyl)amines to yield polysilylbiurets which upon hydrolysis yield polybiurets.